depends on the shell you use, for e.g. bash it is
"export PKG_CONFIG_PATH=/whatever/path"

Well, I get this:
$ echo $PKG_CONFIG_PATH
/opt/gnome2/lib64/pkgconfig:/opt/gnome2/share/pkgconfig:/usr/share/pkgconfig:/usr/lib/pkgconfig
$ for i in `echo $PKG_CONFIG_PATH | tr ":" " "`; do

echo $i
find $i -name "*dbus*"
done

/opt/gnome2/lib64/pkgconfig
/opt/gnome2/share/pkgconfig
/usr/share/pkgconfig
/usr/lib/pkgconfig
/usr/lib/pkgconfig/dbus-1.pc
/usr/lib/pkgconfig/dbus-glib-1.pc
But I'm not sure what I'm looking for or where to look - was that any help?
mhr